Undertale Worlds
by thederpypikachu98
Summary: What happens when Gaster decides to give Sans a DT serum? Why is Sans so powerful? And why does Sans have a hidden lab? Join Sans as he readjusts to his new friends and the worlds around him. SPOILERS FOR UNDERTALE! I DO NOT OWN ANY OF THE AUS EXCEPT MY OWN! DISCONTINUED
1. Chapter 1

**A/N: I'm back!**

 **So today at school I basically did what I call 'Naruto-ing.' That is, pay attention until we were sent off to do our individual work and speed through while also trying to get everything right, then work on writing fanfics. Basically how I write my fics: just go back and forth between electronic and paper and transfer it all here when I get enough for a chapter or two, or just transfer it and edit it.**

 **So not only did I write this, I edited most of it. And I'm posting a lot because I have nothing else to do and I wanted to start a fic with just a bunch of one- or two-shot and then some stories with five or so chapters. Might switch it to five-shots (is that a thing?) and fewer. I don't know.**

 **I can make this a single story or a bunch together. It's all Undertale, so if you guys want me to try writing about a certain AU or story then let me know!**

 **DISCLAIMER: I DO NOT OWN UNDERTALE OR ANY CHARACTERS MENTIONED! I ALSO DO NOT OWN ANY AU'S MENTIONED! I only own the storyline and any OC's.**

 **NOTE: Frisk = non-binary. Mettaton and Napstablook = male. Monster Kid = female. Chara = virus/demon/I don't know. Don't like it then either deal with it or leave (AFTER READING AND/OR REVIEWING!).**

 **Note No. 2: The story jumps right in. I had no idea how to explain it or anything. Originally it was going to be part of Sans' Secret and The Timeline Battle/War, but I changed my mind when it became more of a totally different idea and now it's a stand-alone story. THIS IS NOT A CONTINUATION OF THE SANS'S SECRET FIC! THIS IS A TOTALLY DIFFERENT STORY!**

 **Note No. 3: Backstory: Post-Pacifist. Asgore still stays at the castle but Toriel has a new home in Snowdin. Frisk lives with Toriel and Flowey never revealed to anyone but Frisk that he was Asriel. Flowey lives with Frisk and Toriel. The barrier wasn't opened and the monsters (plus Frisk) stayed down in the Underground. Also the path to Mt. Ebott was blocked so no more Fallen Children... Gaster has returned and lives with Papyrus and Sans. Story start: Toriel and Frisk were at Asgore's and were coming back. Sans and Gaster were waiting up while Papyrus had his cooking lesson with Undyne.**

 **WOW. Long A/N!**

 **Enjoy!**

Gaster went and poured two cups of coffee, his back to Sans. The former Royal Scientist paused, then turned to give Sans his coffee. "Hey, Sans," the tall skeleton said.

Sans blinked and took the coffee, mumbling "thanks" before draining it in two gulps. He yawned and glanced at the window. "Hey, Gaster, do you know when Frisk and Tori are gett-" his eye sockets widened. The white points disappeared, leaving the empty depth. "Gast-" Sans started coughing violently, so hard he knocked his chair over, spilling to the ground. He crawled towards Gaster, who stood by the counter, who was filled with DETERMINTION.

Sans fell to his hands and knees, blue jacket still wrapped around him. He gasped and coughed, pain blazing through his body. Then he felt a prick on his arm, and looked up to see Gaster pulling a syringe out of his bones.

His vision began tunneling. From a distance he saw Toriel and Frisk burst in, and then Tori yelling and Gaster yelling back.

"Sans!" He heard faintly. Frisk- oh, Frisk, the human child, oh, your genocide route was nothing compared to this!

Oh, god, it hurts, please stop the pain! It burns oh god stop it please, help me!

Sans heard his voice and dimly realized he was the one crying out, tears pouring down his face.

"Sans!" Frisk shrieked.

Sans curled into his side in a ball, crying, oh god it hurts what did Gaster DO?!

The last thing he saw was Toriel frantically calling someone and Frisk throwing their arms around him and Gaster roaring at his own phone.

Then everything went black.

_time_skip_

Pain. Fire through his bones. Then it faded and his body grew warm. He could feel a few things in particular; his soul, something cool gripping his hand, and... warmth. Not fire blazing in his body anymore, but a nice heat, so much better then the coldness he's lived through for years and years and-

Then his sense of touch faded and he passed out.

_time_skip_

Voices now. The warmth was back, and his soul, and the coldness gripping his right hand.

"Gaster, I told you, Asgore and Toriel are-"

"Alphys! You know what I told you!" Gaster.

"But you let Papyrus and Frisk in!" Alphys.

Papyrus. Of course. Sans recognized the cool skeletal hand clutching his own now, it was his brother's. Frisk mustn't be here right now, or they would be right near Papyrus.

"So when is he going to wake up? It's been-" Alphys had lost her stutter, as she always did while in her scientist mode. Sans could recognize that tone anywhere.

Gaster sighed. "His soul is stable and his magic is perfectly unaffected. There is no reason for panic."

Alphys literally growled. "You gave him the DT serum and injected him with pure DT! I really wonder why we all are panicking and everyone is crowding around downstairs?!" Sarcasm dripped from her words.

Sans suddenly moaned inwardly as his senses withdrew. *JUST LET ME WAKE UP!* he screamed silently.

_time_skip_

Taste. Hearing. Touch. Three out of five.

He knew this because he could feel someone pressing something to his lips; water. Then a minute later he heard the sound of... was that ketchup? Really? Why-

Oh.

It was Papyrus he heard earlier, arguing that "SANS SHOULD BE ABLE TO EAT WHAT HE LIKES! IF HE WERE AWAKE HE WOULD WANT KETCHUP SO TOO BAD IF THAT SCREWS WITH YOUR STUPID SCIENCE CRAP!"

And then he was eating ketchup. He laughed inwardly. Ketchup. After a second there was more water, then nothing as his senses faded yet again.

_time_skip_

Smell. Taste. Hearing. Touch. Finally, soon he would wake up! The darkness was depressing and he wished more then ever to be able to joke around with his brother and friends.

He smelled the usual scents of his room; with a sharp, clean sort of sting to it. Gaster and Alphys. Again with their stupid lab equipment?

Then it faded again. He wanted to punch something. *Let me wake up!* he screamed silently.

_time_skip_

Light.

Sans now saw the darkness getting lighter. Not by much, but enough that he almost started crying in relief. Not like him, but this was a special case.

And his senses came back. They didn't leave. Only his sight stayed just out of his grasp, slowly lightening the darkness as time went on.

Later he suddenly realized he could feel his whole body, not just the warmth and Papyrus' absent hand.

"GOOD MORNING, BROTHER!" Papyrus' voice suddenly said. Sans guessed he had just entered the room. "I DON'T KNOW IF YOU CAN HEAR ME BUT I'VE BROUGHT SOME OF TORIEL'S LOVELY BUTTERSCOTCH PIE. ITS QUITE DELICIOUS!"

Sans heard Papyrus move around awkwardly and sit to his right. A moment later, something warm touched his mouth. Papyrus was holding his right hand with his left and giving him pie with his right.

Sans struggled to move, to blink, ANYTHING. He gasped silently, then took a deep breath.

He concentrated on his right hand, the one that was clasped in Papyrus' hand. He strained and strained and-

Until, miraculously, Sans managed to squeeze Papyrus' hand before mentally collapsing with exhaustion.

Papyrus gasped and Sans heard the fork hit the ground. "Br-brother?" His brother whispered, not yelling for the first time in who know how long.

Sans felt Papyrus throw his arms around him, sagging with relief inside. The darkness was lighter, sure, but it was still an extremely dark gray.

He felt himself slip into sleep once again.

_time_skip_

Light.

Sans' eyes flew open and he sat up extremely fast, chest heaving.

Beside him, Papyrus whispered, "S-sans?"

Sans stared at the base of his bed, breathing heavily. Something was different. His eyes were wide, silent.

"Sans!" Gaster gasped from the other side of his room.

Fury filled Sans and he thrust an arm at Gaster. A Gaster Blaster appeared in front of him and fired, firing a blast bigger then he's ever summoned before. Gaster barely had time to teleport out of the way before the blast hit the wall and exploded out into Snowdin.

Sans froze as he stared at his hand. What-

"Sans, calm down-" Gaster began but Sans yanked the covers off his legs and stared at his very human, very NOT skeletal legs and feet.

"GastER I AM GOING TO KILL YOU!" Sans bellowed and leaped at the scientist - or tried to, at least. He more or less fell out of bed and face-planted on the floor. "YOU SON OF A-"

"SANS!" Papyrus yelled. He picked Sans up under his arms, holding him up in the air. "CALM DOWN!"

Sans was breathing heavily, and glanced up to see frosty white hair falling across his forehead. He still wore his usual black shorts and white t-shirt. His pink slippers were by the door and his jacket behind Papyrus' chair.

Sans let out a groan. "Gaster I swear, you better tell me what you did or I swear you're gonna have a bad time!"

Gaster swallowed. "I slipped a DETERMINATION serum into your coffee. You started changing right there. It was too fast, so I injected pure DETERMINATION into you."

Sans began struggling. "Paps, put me down-"

Papyrus set him down and immediately, Sans was grabbing his blue jacket and shoving his feet into his slippers. "BROTHER, WHERE ARE YOU GOING?"

Sans whipped around. "Away from Gaster."

He teleported away before Papyrus could grab him.

Sans stumbled as he reappeared in his lab, shocked at how easy his teleporting had been. He managed to move to the sets of drawers in his lab and braced himself against the shiny silvery counter.

A white-haired, pale blue-eyed boy stared back at him, about sixteen. He had Sans' blue jacket and his white shirt and even that hint of laughter in his eyes.

Sans stumbled back and sank to the ground with his back against the wall with his head in his hands. He couldn't do this, he can't go back-

He can't go back to his past. The machine is broken, he can't use it. That Timeline's gone anyways. He'd never get home. He was stuck here, and he had finally accepted himself as a skeleton and a part of this Timeline when that stupid Gaster had to go and change him back.

Sans buried his head in his arms. "Why, Gaster?" He muttered. "Why do you have to do this to me?"

After a moment, he curled up against the wall, pulling his jacket closer around himself.

 **Ending A/N: I probably shouldn't have put so much into one chapter.**

 **BUT FUCK HAVING A FIFTY CHAPTER FANFIC I WANT A NICE 5 TO 10 CHAPTER FIC!**

 **Also yeah. I don't own the cover but I found it online while looking for a good image of Human!Sans, and I don't know who owns it. I DO NOT CLAIM TO OWN IT! And also Papyrus will not become a human. And Frisk's not turning into a non-binary, bi-pedal goat-monster. Sorry.**

 **If you have any special AU's you want in this for later on (hint, hint) then feel free to let me know and I will research them or figure out how to distinguish future characters from each other. (Hint, hint.)**

 **Also I'm listening to the Sans Stronger Than You on loop. And... maybe the entire Undertale OST. *cough cough Megalovania cough cough***

 **Thanks for reading HumanUnderTale! (Also that's a really weird name but I have no idea what to call it.)**

 **:3**


	2. Chapter 2

**A/N: Chapter 2!**

 **I managed to figure out a quicker way to transfer this from paper so here's chapter 2. Sorry if I've been posting a lot, but I've sort of figured out a schedule!**

 **I'll post one chapter of Sans' Secret and The Timeline Battle/War a day, maybe every other day once I finish posting what I already have done. I can also update Pokémon: Vinoa Region every other day because I don't have as much pre-written for that. And finally, I'll post as much as I have written for these mini fics/stories(whatever these are)each day because why not?**

 **Thanks to InkSans9542nd for your review! I'll definitely put Ink in when it comes to that part. It'll be a couple chapters though.**

 **DISCLAIMER: I DO NOT OWN UNDERTALE OR ANY CHARACTERS! I DO NOT OWN ANY AU'S OR CHARACTERS! I only own the storyline and and OC's.**

 **NOTE: Frisk =non-binary; Chara = virus/demon/idk. Don't like it then either deal with it or leave (PLEASE READ/REVIEW FIRST!)**

 **Enjoy!**

Voices drifted past him in his dreams. "Sans! Where are you?" Toriel cried out.

"Sans?!" Frisk.

"Sans, please! Where'd you go?!" Undyne.

Even Flowey. "Smiley! Where the heck are you?!"

The one that tore him up he most was Papyrus' shouts. "BROTHER?! PLEASE, COME HOME! WE MISS YOU! I-I miss you..." Papyrus trailed off, and the sounds of sobs woke Sans.

He opened his eyes and touched a hand to his cheek. Tears. He had been crying.

Sans stood, still wobbly. He was taller as a human than he was as a skeleton, and his limbs were longer as well. He wasn't used to this; he hasn't been a human in years.

Sans sighed and looked around at his lab. The blueprints caught his eye and he managed to get closer to look.

The machine that would have destroyed the virus. The virus known as Chara, the first Fallen Human. The machine that would have saved his home Timeline and made sure that Chara would never make it to any other timeline. The machine that had failed and destroyed his timeline instead with him as the only survivor. The machine that sent him to the only other timeline who already didn't have a version of Sans because that was the change in the timeline. The machine that brought him from HumanTale to Undertale.

The machine known as the Timeheart.

_flashbacks_

The plan consisted of Sans using the Timeheart to capture seven human souls from another Timeline to allow one of the HumanTalers to become a god and destroy Chara the virus. But the virus had struck too soon and Sans barely fled with his life, running away from the virus as it corrupted his home and friends.

He woke up in a snowy forest with the remains of the machine beside him. It was still intact, but the screen was shattered and the words 'Undertale' flashing on the remainder of it. He was cold, so very cold.

He hid the machine and stumbled to a door at the edge of the forest. Seconds later, a human peeked out, no older then eight, with wide bluebell eyes and black hair.

Sans had take the humans' soul for a moment and used some of their INTEGRITY to transform himself into a monster; a skeleton. He returned the soul and left the human girl there in the snow.

Seconds later something glowed behind him and as he hid again in the trees, he saw the girl create a shining star; a SAVE point. She pressed SAVE, then CONTINUE.

His form was permanent. Sans would make sure of it.

As time passed, he got used to the new timeline, and even saved a young skeleton child from the attacks of the second human to pass through, the GREEN soul. He named the child Papyrus and raised him, claiming to be Papyrus' older brother.

Then Gaster appeared, moving to Snowdin to continue his studies, and became their father figure, hiding from humans and training Sans and Papyrus. Gaster was the Royal Scientist for King Asgore, studying DETERMINATION and the human souls.

Until one day when Sans woke to find Gaster was no longer the Royal Scientist. He was to train an apprentice, Alphys, a young dragon-like monster who showed huge amounts of interest in the studies of DETERMINATION. Sans was always different from the other monsters; sure he joked a lot and pulled pranks, but he was lonely. Papyrus didn't know about Sans' past, nor didn't the younger skeleton know that Sans wasn't his true brother.

Then Frisk fell, and the resets began. No other child could RESET, only SAVE. And the only other creature he knew could RESET...

And then the genocide began. Frisk was different the moment they walked out of the Ruins, flipping a knife in their hand and with red eyes and pink cheeks. They wore a green and yellow sweater that was... that was coated in a thin layer of dust.

Sans fled his post, hiding the Timeheart and teleporting to the castle. He only made it to the judgement hall, where the child caught up to them.

"Hello, Sans," the child sweetly said. "Remember me?"

Sans just attacked; having once been a human and still having a sort of human soul, he was powerful, the hardest type of boss monster in Undertale to beat.

The child stared at him until their smile and eyes began to almost melt. "DO YOU REMEMBER MY NAME?"

Sans attacked. The child dodged and smiled evilly. "DO YOU REMEMBER HOW I CORRUPTED YOUR FRIENDS? HOW I DESTROYED YOUR TIMELINE?" They slashed with their knife and Sans dodged it. "I'M THE ONE WHO KILLED THEM, SANS. I'M CHARA!"

And when Chara finally killed him, he whispered, "I'm sorry, Frisk."

And turned to dust.

What felt like moments later, he was awake, sitting up on the same sentry post he woke up at when Frisk reset.

Sans prayed every reset that Frisk would exit the Ruins, not Chara, the virus that somehow made it to Undertale. And when Frisk peeked out, his legs would give way and he would slide to the ground in relief, then rush after Frisk to confront them the same way every single reset.

But this time, the Pacifist child didn't reset.

This time, they had shattered the reset button.

And Sans was a human again. Thanks to that GOD DAMNED GASTER!

_present_time_

Sans stood and stared at the blueprints for the Timeheart. He glanced back at the covered machine, then sighed.

He laid his jacket on the counter and yanked the sheet off the Timeheart. It was the same. But he needed it to be different, just this once.

Sans crawled under the machine and started working.

_time_skip_

Papyrus looked at his and Sans' house. He didn't want to go back there, not without his brother. The tall skeleton sighed and trudged around to the back of the house of his shed that he'd once used as a human trap but was now filled with his favorite things; spaghetti, pictures of himself and Sans, bones and various other items.

But when he looked up, he realized he had gone the wrong way. The other side of the house was Sans's shed, and Papyrus respected Sans' privacy.

Papyrus had never bothered to check whether there was an actual shed there or not. Because there was only the corner of his, poking out around the side of the house.

There was a door though. Papyrus opened it and heard a bang and a grunt from down a flight of stairs.

Quietly, Papyrus closed the doors and snuck down the stairs. It opened into a lab of sorts, with-

Oh, god. Sans was examining a blue poster tacked on the wall, then somehow managed to clamber underneath the machine. There was a bang and a screech of metal. Then a loud thump and a crooked pipe came flying out from under the machine.

"S-Sans? What- What's all this?" Papyrus asked.

Sans evidently dropped something and Papyrus heard it his something. "FUCK!" Sans yelled. With a tiny *pop*, Sans appeared in front of Papyrus, rubbing his elbow. His frosty hair was disheveled and his white t-shirt was stained with grease and god knows what. His slippers were cast against the wall, so he was barefoot and his jacket was on the side counter. "P-papyrus? How-"

Papyrus pulled his brother into a hug. "Sans, did you not hear us calling? We were all so worried!"

Sans glanced back at the machine once Papyrus let him go. "Uh, yeah... sorry about that, bro." The kid turned and rummaged in a drawer, mumbling something about a time difribulator.

Papyrus squinted at the blue poster. "You can read this, Sans?"

Sans pushed past him and climbed back under the machine. "Yeah," his voice echoed from under there.

Papyrus looked at the machine. "It looks... interesting," he said slowly.

Sans teleported back out, startling Papyrus. "Ah, I don't care if it doesn't look good. I just need it to do its job right." Then he frowned, cheeks flushing slightly. "Hey, Papyrus?"

"Yes, brother?"

Sans swallowed. "C-could you make sure everyone knows I'm okay? Don't tell them where I am though! And... maybe could you bring some... some food?"

Papyrus blinked. Then he smiled broadly. "Of course, Sans! I will always help you out!"

Sans grinned. "Thanks, Pap." Then the now-human teen dove back under the strange machine, tinkering with something.

Papyrus left Sans' lab thing and reentered the house. Everyone looked up at him expectantly.

He cleared his throat and resumed his overly cheerful personality. "I HAVE FOUND SANS!" Papyrus declared.

The group of monsters and Frisk swarmed him. "Where?!" Asgore asked.

"Is he okay?!" Toriel begged.

"Does he have enough food?" Frisk asked.

Papyrus raised his hands. "Sans asked me to tell you all that he is okay, just that he is currently occupied. He just wanted some food and then I'm taking that to him. I might keep him company for a bit."

Toriel ran for the kitchen. "I have a pie he might like!"

"And I can make him tea!" Asgore put in.

"He likes burgers," Frisk said. "Get him Grilbys' and some of Toriel's pie and he'd be happy. And since he's... well, get him some tea or something so he's not just drinking ketchup."

Papyrus went and gathered a few extra t-shorts, shorts, a blanket and a pillow for Sans, stuffing them in one of Sans' many unused bags. Then he gathered his own pillow and blanket. Collecting Toriel's pie and Asgore's tea, Papyrus managed to make it to Sans' lab.

"Sans! I brought you a pillow and a blanket, and some more clothes. I also have some of Asgore's tea and Toriel's pie." He set the food on the counter. "I also can get you a burger from Grilbys' if you want."

Sans teleported out from under the machine. "Wow, Papyrus. Th-thanks."

Papyrus grinned. "You're weaker as a human! I have to take even better care of you!" He waved, and began walking to Grilbys', leaving Sans to do whatever in that lab of his.

He entered the building. Grilby looked up and stopped wiping down his counter as Papyrus walked up to him. "HELLO, GRILBY!" Papyrus grinned.

"Hello, Papyrus. You haven't been here in a very long time! What can I get for you?"

"MAY I PLEASE HAVE A BURGER WITH A NORMAL AMOUNT OF KETCHUP AND FRIES?"

"Certainly, Papyrus." Grilby handed him the meal after a moment. "Now, who's this for? I haven't seen your brother in here for a while. Is he okay?"

Papyrus continued smiling, but it was a bit more forced now. "OH SANS IS PERFECTLY FINE! I'M JUST GETTING A BURGER AND FRIES BECAUSE I HAVEN'T HAD ONE IN A WHILE. HAVE A NICE DAY, GRILBY!" Papyrus quickly scooped up the food, paid Grilby, and made it to Sans' lab in record time.

Sans was still working, but now he was poking the shattered screen. "Hey, Papyrus!" He greeted.

Papyrus handed Sans his food. "How'd you know it's me?"

Sans took a huge bite of burger. "Because you're the only one who knows this place is here," he said after swallowing.

Papyrus took a nap while Sans ate. His brother had an even larger appetite now that he was human, although he seems to realize that too much ketchup was actually a thing.

_time_skip_

Sans finished his burger and put all the wrappers into the paper bag. Papyrus snored softly beside him, clutching his pillow and blanket as he slept.

Smiling, Sans then stood and looked towards the Timeheart. He moved to the machine and began working.

 **Ending A/N: So I'm probably going to add one or two more chapters tonight. It only takes me a while when I have to type it in from my fanfic notebook. Editing doesn't take very long for me.**

 **Chapter 2! I'm going to have to research more AU's later which already include classic Sans, Ink, and a couple pre-chosen ones already. If you want any AU's in for later, please let me know!**


	3. Chapter 3

**A/N: Chapter 3~**

 **Yes. I am posting a lot of this story. But that's what I have written so far and so why not?**

 **Thanks to Ben10extreme for reviewing! I'll see if I can fit Geno in - now that I think about it, I have a pretty good idea already.**

 **DISCLAIMER: I DO NOT OWN UNDERTALE OR ANY CHARACTERS! I DO NOT OWN ANY AU'S OR CHARACTERS! I only own the storyline and my OC's.**

 **NOTE: Frisk is non-binary. Chara... well, Chara is essentially a virus/demon/thing so idc?**

 **Enjoy!**

Sans stared at the shattered remains of the screen. It was the last part that was broken... he hoped.

Carefully, Sans pried the larger, less broken shards out and laid them on the counter before working on the smaller bits that poked out. After nearly stabbing himself with a particularity sharp piece, he summoned a tiny bone to wiggle them loose and caught them in his palm. Depositing those in the pile of scraps and twisted metal in the corner, Sans examined the edge of where the screen had been.

"It might work..." he muttered, and teleported to his bedroom. Luckily, no one was there, and he was able to find the old TV in his closet. He'd never used it, and now Sans was glad he hadn't. A few quick bones between the screen and edge, and the screen fell out into his hands. Teleporting back to his lab, the remains of the TV went right to the trash, and the screen fitted perfectly into the machine.

Sans held his breath and pressed the 'ON' button. For a second there was nothing, then the machine beeped once.

"YES!" The Timeheart hummed and booted up. Lighting up, the screen flashed once and displayed the word 'Undertale' in bold letters. "YES! IT WORKS!" Sans tackled the still-sleeping Papyrus, who was awoken by an excited human teenager(give or take).

"SANS!" Papyrus cried. "I WAS SLEEPING!"

Sans laughed, and pointed at the Timeheart. "It works! I fixed it!" He teleported over to the drawers and yanked them open, piling a badge, a photo album, a picture, and other random items on the counter. Teleporting around the room, Sans yanked blueprints off the walls and shoved them into the pile. With a *pop* he was gone, leaving Papyrus looking bewildered, only to return half a minute later with a worn blue backpack. Not even really paying attention, Sans' eyes glowed blue as he used magic to stuff the backpack with the pile of items and zip it shut.

"Sans? Fixed what? What does it do?" Papyrus squinted at the machine. He stood and neatly began folding his blanket.

Sans used magic to snatch it from him and speed fold it, surprisingly nicer then Papyrus. The blanket was set on the ground and then Sans was grabbing Papyrus' hand and they were teleporting.

With a louder *pop* and a crack as well, they were in their living room. Papyrus fell onto Undyne's lap, who was sitting on the couch. Their combined weight, plus Asgore and Toriel's who were also crammed onto the couch, caused the old couch to give way. Sans, of course, landed on his feet in the middle of the room.

Everyone stared at them, except for Undyne who was busy shoving Papyrus off her lap.

Sans swallowed. "Okay, I'm going to run through this as fast as I can. Frisk has a virus inside of them. You need to guard them and make sure the virus doesn't escape. Asgore, Toriel, it may come out and try to convince you to let it go. If it does, it will jump to your body and spread. Whatever you do, DON'T LET IT OUT." He counted off on his fingers. "That's number one. Two, I need to go somewhere and you can't come with me. It'll screw with the whole universe and the only reason I'm okay is because I've been doing small bits for my entire life. Three, Gaster. Stop fucking with my life and if you try to give anyone else the DT serum, I will personally come find you when I'm back and you won't like it. Four, Papyrus, you and Undyne need to evacuate the town. Asgore, move the human souls somewhere safer and hide their bodies." Sans stared at the shocked group. "Got it?"

Papyrus, Undyne, Alphys, Gaster, Asgore, Toriel, Flowey, Frisk, and surprisingly, Muffet, stared at him in shock.

Sans groaned. "Gaster, I swear to god! Do I have to explain every last detail?!" He sighed impatiently. "Guys, it's me. I'm Sans. Gaster here decided it would be a great idea to give me a DT serum, then inject me with pure DETERMINATION. As you can see, it turned me back to my old body. There. Now I've gotta go. Any questions?"

Gaster spoke. "Did you see your 'old body?'"

Sans pale blue eyes widened. "Shit. Well, that's a story for when I get back. Pap, don't tell them where I am. Or what I was doing." His eyes blazed with magic and he teleported back to his lab.

Lightning fast, his fingers flew across the screen, selecting specific commands and changing settings. He pressed a couple buttons, flipped a switch, then slammed his fist into the 'START' button.

The Timeheart hummed louder. "C'mon! Work!" Sans hissed.

Behind him, the door was kicked open by a red boot. "SANS!" Papyrus yelled. Behind him, everyone but Frisk and Gaster poured in.

Sans whipped around, frosty hair messy and his eyes wide, sparking with magic. "Papyrus! No!"

"Sans?! What are you doing?!" Alphys cried. "You're-"

Sans growled, "I KNOW WHAT I'M DOING!" He threw his hand up and felt his magic blaze. A wall of bones burst out of the ground, causing his friends to scramble back. It left gaps small enough that they could see him frantically smashing buttons and changing settings as fast as he could. "You guys weren't supposed to be here!"

The humming grew louder. Alphys shrieked, "SANS!"

Sans glanced at them over his shoulder. "Sorry, guys. I have to do this."

He turned their souls blue, and levitated them in the air. The monsters struggled, eyes wide and fearful as they realized that they couldn't change their souls back to normal, that Sans was way more powerful then they had thought. Carefully and quickly, he dragged them up the stairs and out of the lab. He released their souls and used his magic to slam the door, locking it. They were slamming their fists on the door, yelling.

Sans punched the 'CONTINUE' button.

The machine flashed one word on the screen before his vision went white.

'UnderSwap.'

 **Ending A/N: Cliffhangers are both torture and fun to write.**

 **I loved how originally this chapter was going to be part of Pokémon: Vinoa, but with my other characters replacing Sans and the Undertale crew. But then I remembered my ending for that and realized it wouldn't make sense, so I stored it away for another story. And here it is!**

 **So you might be able to predict what's going to happen. Or you might not. Who knows?! If you have any AU's you want, feel free to let me know! Sans, Ink, Geno and Blueberry are going to be in the next chapter (hint, hint) and I'm going to add more.**

 **I'm so glad that people like this fic and my others. I love that it's been just about two days, give or take a few hours, since I posted my first fic and people like liked it. Thank you so much! Every time I got a review it literally made me smile so much and made me so happy that people read my fics!**

 **Let me know if I make any mistakes so I can improve, and let me know what other stories I could start planning for after this one! I've got a couple ideas but it'd be cool to hear what people want.**

 **Chapter 4 tomorrow, as well as the next chapter for my other Undertale fanfiction, Sans' Secret and the Timeline Battle/War. (I say the battle/war because it's kinda both that I have planned.)**

 **Byeeeee~!**


	4. Chapter 4

**A/N: I did some research today on some of the AU's that will be in this story. I realized a couple things.**

 **1)** **Classic Sans. I'm letting you all know that the human-turned-skeleton-turned-human character is NOT Classic Sans. He's from HumanTale, and his timeline was corrupted by Chara, the living virus that wants to destroy all timelines. He fled HumanTale as Chara attacked and made it to Undertale, using the Timeheart, like it was explained in Chapter 2. He used the INTEGRITY of a human child to transform himself into a skeleton. Also, it will be addressed in the fic today.**

 **2) I won't have Sans explain his story to every single new character he meets today. It'll get boring to both read and write and also make each new encounter extremely long.**

 **3) Yes, like I said, SwapSans(Blueberry/Blue), GenoSans(Geno), FellSans(Fell) and InkSans(Ink) will be in this fic. I'll only be able to fit two of them in this chapter, maybe three, and the rest or a couple more next chapter. I won't use all the AU's because then it would be really hard to make sure all of them got to interact and there's a ton. When I looked for a HumanSans, it didn't come up as a character on the Fandom Wikia, only theories and different images. CLASSIC SANS WILL BE IN THIS FIC! OTHERWISE INK COULDN'T EXIST!**

 **Thanks to the guest who pointed that out. I hope I sorta explained it and if I didn't it definitely will be at some point during the fic. I'm shooting for about seven chapters and probably am going to change this to its own fic if you guys want.**

 **NOTE: Frisk is non-binary and uses them/they/their pronouns. Chara... well, it/she/he/they is a virus/demon thing so I have no idea. Probably also going to be using they/them/their pronouns.**

 **DISCLAIMER: I DO NOT OWN UNDERTALE OR ANY CHARACTERS! I DO NOT OWN ANY AU'S OR ANY CHARACTERS! I only own any OC's I made and the storyline.**

 **Enjoy!**

Sans stumbled and fell to his knees, landing in cold, wet snow. The whiteness faded from his eyes and he blinked twice.

The Timeheart was tucked under a snow-covered tree next to him, displaying the word 'UnderSwap.' Lights poked through the trees, and Sans could see buildings he recognized.

"YES!" He cried, leaping to his feet and checking the Timeheart. His feet and legs were cold, but he didn't care. It worked!

Sans teleported closer to the UnderSwap Snowdin. Everything looked the same, except for the monsters dressed slightly different. He crept closer to what would have been his and Papyrus' house, and peeked out from behind it. No one was looking, so he ran for the front door and yanked it open. Quickly zipping inside and accidentally closing the door a little harder than he liked, Sans listened for any sound of the UnderSwap Papyrus or the UnderSwap Sans.

An extremely cheerful voice yelled from upstairs, "PAPYRUS? IS THAT YOU?!" Someone opened a door on the second floor and footsteps moved towards the stairs. Sans dove for the couch and squeezed himself behind it, thanking the fact that his human form was slightly thinner then he skeleton form. He peeked out cautiously.

A short skeleton with starry blue eye things in his eye sockets hopped down the stairs. He had a bright blue scarf-thing tied around his neck, and wore a gray shirt with a blue hem. His boots were a matching blue. The UnderSwap Sans looked around as he moved towards the door and stared out at Snowdin. Returning, he started towards the kitchen.

Sans watched the skeleton go into the kitchen and started trying to wiggle out of his hiding spot. After a second of struggling, he gave up and teleported out, with a small *pop.*

The skeleton poked his head out of the kitchen before Sans could move out of the middle of the room. Sans' eyes widened and he froze. "Oops," he muttered.

SwapSans' jaw dropped and his starry eyes grew wider. "A HUMAN?!" Then he scrambled out, brushing nonexistent dust off his clothes. "HUMAN! I AM THE MAGNIFICANT SANS!"

Sans heard a thump and a yell of pain from upstairs. "SANS?! WHO ARE YOU TALKING TO?!" A voice called.

"Who's that?" Sans asked the smaller skeleton, who grinned.

"THAT'S JUST MY EXTREMELY LAZY BROTHER, PAPYRUS! HE-" the skeleton was cut off as Sans dove forward, and just as he grabbed SwapSans' arm, teleported.

Sans was already adjusting the settings on the Timeheart to allow two travelers by the time SwapSans had caught his bearings. "WHERE ARE WE?!" He cried, still shaking his head from the experience.

"Outside of Snowdin. This is UnderSwap, right?" Sans barely looked to register the fact that the skeleton had nodded. "Cool. Mind if I call you Blueberry? Or Blue? It's gonna be hard to figure out names once..." he trailed off and finished changing the Timeheart. Sticking his hand out, he grinned at Blueberry. "I'm Sans."

Blueberry shook his hand excitedly. "SURE! BUT IF YOUR NAME IS SANS, THEN WHY IS MINE ALSO SANS?" He sure could yell. Just like Papyrus.

Sans chuckled. "Welp, that's because..." and he was off, chattering about how he got here and why and the whole story. When he was done, Blueberry was staring at him with even wider eyes. "So I need your help. And the help of other Sans-es in other Timelines."

Blueberry gasped, looking between Sans and the Timeheart. "THAT'S BOTH SAD AND AMAZING!" He struck a pose, which Sans noted, was similar to Papyrus' silly poses. "I, THE MAGNIFICANT SANS, WILL ASIST YOU!"

Struggling not to laugh at his new friend's antics, Sans flicked a switch and pressed a button. "Good."

Both of their visions went white, and Sans got ready to possibly teleport if the need arose. After half a second, he landed in snow again and his vision returned to normal. Beside him, Blueberry stumbled.

Like before, he could see Snowdin's lights up ahead through the trees. The Timeheart beeped twice, and a message scrolled across the screen. Sans scowled as he read it.

"WHAT DOES THAT SAY?" Blueberry asked after regaining his balance.

"It needs a cooldown. There's enough to get three of us back to Undertale, if FellSans decides to come." Sans put his hand on Blueberry's shoulder and teleported them to this timeline's Sans' and Papyrus' house. It looked essentially the same, but with red and black lights. "Stay quiet and get ready to run if we're attacked," Sans whispered. They moved to the front of the house and Sans out a hand on the door to open it.

Two *tings* and they were thrown into the air, souls a reddish-blue. Blueberry started to scream, but Sans summoned a skeletal blue hand that slapped itself over Blueberry's mouth. They started falling, and Sans struggled. After a gut-wrenching second he felt his soul escape the magic.

Throwing his hand out, a Gaster Shield appeared under their feet and halted their fall. Blueberry's starry eyes widened as white and red bones appeared around them. Sans finally spotted their attacker below and in front of them.

The skeleton wore a black jacket with more fluff then Sans' had, left eye blazing red. His teeth were sharp with a single gold one, and he wore a red shirt under his jacket. Red sneakers adorned his feet, and his black pants had gold stripes running down the side. Glaring at Sans and Blueberry, he flicked his right hand to the side.

The floating bones around them sprang into motion, bulleting right for them. Sans pulled Blueberry closer and teleported to the ground, breaking the blue magic away from Blueberry's soul. The attacking skeleton blinked and his attacks disappeared.

"Hey, you two! Who the hell are you, and what are you doing, trying to bust into my house?" he growled, stomping closer. "How'd you break out of my-" The skeleton's eyes darkened and his eye once again blazed with red magic. "A human."

Sans out his hands up. "Wait! My name is Sans. I'm from HumanTale originally, now I live in Undertale. This is Sans from UnderSwap, but he's okay with being called Blueberry. We weren't trying to break in, we wanted to see if you were home."

UnderFell Sans laughed. "Su~ure. We'll go with that." Then he looked at his house. "You know what, fine. I'll listen. But let's not stay here. My stupid brother'll probably be pissed that I haven't tried to capture you."

Sans carefully grabbed hold of both Blueberry's scarf and UnderFell Sans' jacket. He teleported them back to the Timeheart.

FellSans narrowed his eye sockets at the sight of the machine. "The hell?"

Sans sighed and explained. Blueberry occasionally jumped in, adding random comments. As he spoke, FellSans' face actually seemed to soften, and he looked between the two after Sans finished.

"So basically you're trying to raise a sort of army of alternate versions of the same guy so you can fight an evil virus that's already destroyed your original timeline and made it to your current timeline," he summarized.

Grinning, Blueberry bounded up and down. "YEP!"

FellSans shook his head, then glanced at them. "Fell. Call me Fell. You say you've got a bunch more Sans-es to find and calling them all by their timeline or some aspect of their personality seems like a pretty good idea." He waved a hand in the general direction of the Timeheart. "Just start the damn thing."

Sans grinned, and slapped the button. The Timeheart brightened and their visions went white yet again. Sans was starting to get used to the brightness.

His feet landed on firm, hard ground this time, and his vision became normal. Almost immediately, skeletal arms yanked him into a hug.

"SANS!" Papyrus cried out. "YOU'RE BACK!"

Sans coughed. "Can't... breathe..." After being released, he caught his breath and stared up at Papyrus. "Hey, bro."

Papyrus beamed. "EVERYONE'S UPSTAIRS. THEY WANTED TO STAY DOWN HERE BUT I MADE THEM STAY OUT SO IT'S LIKE THEY STILL TECHNICALLY DON'T KNOW ABOUT YOUR LAB."He looked behind Sans. "WHO ARE YOU TWO?"

Blueberry bounced forward, dragging Fell behind him. "HELLO! I AM THE MAGNIFICANT SANS! BUT YOU CAN CALL ME BLUEBERRY!"

Fell rolled his eyes, but smirked. "Also Sans, but Fell's the nickname."

"Blueberry, Fell, this is the Undertale Papyrus. Everyone else apparently is upstairs." Sans glanced back at Papyrus. "The T- uh, the machine just needs a cooldown."

The younger skeleton nodded. "ALPHYS REALLY SEEMED INTERESTED IN YOUR LAB BUT UNDYNE MANAGED TO CONVINCE HER TO WATCH SOME SORT OF ANIME INSTEAD." He grabbed Sans' hand, and Sans shivered slightly at the cooler touch. Blueberry and Fell followed them as Papyrus practically dragged Sans back to the house. "SANS HAS RETURNED WITH SOME FRIENDS!" Papyrus declared, bursting in.

The couch was gone, and a bunch of chairs replaced it. Asgore and Toriel looked up from their conversation with Gaster. Undyne smashed the remote and somehow hit the pause button among all the other ones, Alphys gasping. Flowey was watching Muffet prepare some sort of pastry in the kitchen.

Sans felt his cheeks warm, and was pretty sure he was turning red.

"SANS!" Everyone sprang up and crowded around him, talking and asking questions.

Eventually, Sans got them to quiet down and introduced Blueberry and Fell. Then he finally told his story.

Everyone started talking and yelling. Sans winced at Papyrus' heartbroken face.

Fell grinned, gold tooth flashing as he opened his mouth to talk. A huge *pop* interrupted him as a paint-splattered skeleton teleported into the room, startling everyone into summoning attacks.

The skeleton glanced around until he spotted Sans. He seemed to relax slightly. "Oh, good, you've come back." Striding forwards, the skeleton stopped in front of Sans and held out his hand. "Glad you actually made it here all those resets ago. The other Sans offered his timeline as a refuge for you to fix the Timeheart back before the TIMELINE RESET. He's been hanging out with some of the Sans-es who stay in the anti-voids."

Sans swallowed and slowly shook the pain-splattered skeleton's hand. "Um, who are you?"

The skeleton face palmed. "Sorry, forgot my manners. My name is Ink."

 **Ending A/N: As Blueberry would say: Mweh heh heh!**

 **So chapter 4! Hope you guys like it, because originally it was nothing like this. I don't add a chapter until I think it's good, and before it made no sense whatsoever. This should be better.**

 **And CLIFFHANGER!**

 **Seriously though, thank you guys for reviewing or favoriting or following. Please let me know of any other AU's I can add! I have one in mind, plus the ones I listed in the first part of the A/N.**

 **Chapter 5'll be out tomorrow!**


	5. Chapter 5

**A/N: I really love writer's block #sarcasm.**

 **So this chapter's a bit different from the other ones. And you'll see why.**

 **DISCLAIMER: I DO NOT OWN ANY OF THE UNDERTALE CHARACTERS! I DO NOT OWN ANY OF THE AUS BESIDES MY OWN!**

 **Note: Frisk is non-binary and uses they/them/their pronouns. Chara is a virus/demon/ghost thing so idk what they are, they use they/them/their pronouns.**

 **Enjoy!**

_flash_back_

Sans chuckled as Frisk chattered to his brother about something while they cooked. The human child actually was getting Papyrus to improve his spaghetti, and truly other foods. It was actually edible now.

He could hear Undyne yelling at her video game, Asriel laughing and Alphys cheering them on. Asgore was out back, rummaging around in his garden, while Toriel was baking a pie in the kitchen. Muffet and Mettaton were over, talking about some sort of deal in the other room.

A year and a half of happiness. A year and a half with his brother and best friends.

Sans stood and entered the living room, just in time to see Alphys win at the racing game they were playing. Undyne congradulated her, while Asriel grumbled his way through starting a new round. "so how many rounds have you gone through?" he asked, startling the three gamers.

"I've won three, Asriel's got two, and Alphys just won her fourth," Undyne reported.

Asriel glanced at the kitchen. "Is mom's pie done yet?"

"i dunno, kid."

Asriel scowled at the pun.

There was a knock at the door. Sans teleported to it and peered out through the window. His eyesockets narrowed.

A skeleton, with changing eyelights and paint-splattered clothes waited outside. He spotted Sans and brightened.

Sans slowly opened the door. "who're you?"

The skeleton smiled. For some reason, Sans seemed to recognize him. "You're Sans, right? I'm Sans from Inktale, but you can call me Ink. I have a question for you about something we can't talk about here."

He recognized that skeleton because he WAS that skeleton. Minus the paint and the strange clothing, plus the eyelights that shifted every time he blinked, Ink was an exact replica of him.

Sans glanced back to make sure everyone inside was occupied, then stepped out onto the porch, closing the door behind him. "talk."

Ink went off on this huge schpeil about other alternate universes and how he tried to keep them safe and there was some sort of virus threatening to spread and a Sans from what sounded like Humantale needed a world to take refuge in but that meant the Sans already in it needed to give up his place in that timeline and come with him to the anti-void or whatever so the Humantale Sans could recover and figure out a way to fix his machine and then he and Ink would gather more Sans-es and help destroy the virus and-

"woah, calm down. why are you picking me? why undertale? why not some other alternate universe?" Sans asked.

Ink started counting off the reasons. "Undertale is closest to Humantale, exactly the same, in fact, just monsters are humans and humans are monsters. Underfell Sans is too deadly, UnderSwap Sans is too cheery, Gastertale Sans is too serious, even for a Sans, Undershadow Sans' soul is way too different than Humantal's Sans, Aftertale Sans is eternally dying and alone in the anti-void, ScienceSans is way too close to DT epiraments all the time and that's a huge risk, and -"

"okay, i get it," Sans interrupted. "what do we have to do?"

Ink looked at his feet. "Frisk needs to initiate a True Reset. During the amount of time where the world resets, we need to leave the timeline to allow Humantale Sans' machine to think Undertale doesn't have a Sans."

Silence.

"who is this virus?" Sans asked. "why does it exist?"

"That's the thing. We don't know why it exists, but we do know what it calls itself. Chara. It has the same name as the first fallen human in multiple universes." Ink searched his expression. "Please, Sans. Humantale is destroyed, ripped to shreds by Chara. Humantale Sans is inbetween universes now, and if you don't make the decision now, he will be trapped in the void forever."

A soulbeat passed, and Sans nodded. "if this eventually saves everyone, then yeah, fine, i'll do it."

_time_skip_

Frisk gasped as their whole body was encased in blue magic. "Sans!"

"sorry, kid." He made them press the reset button, and hold their hand there. The button twitched and a new word appeared in front of it. "it's to save all the alternate universes."

The button chimed and the world swirled. Ink materialized and used a giant paintbrush to splash blue pain on the ground. They jumped in, and left Undertale behind.

_end_flash_back_

 **A/N: So this was a sort of 'how Classic Sans left Undertale and Humantale/whatever universe it was Sans came to Undertale' thing. Essentially a prequel.**

 **I hope you guys like it!**

 **I use the fandom wikia thingy for research, and if I need any more info I'll be sure to ask you guys!**


	6. Author's Update

AN:

Alright, I hate authors notes as a singular chapter because they make it seem like the story is updated, but unfortunately that is not the case today.

Just letting you all know - and this AN is going up on all of my stories - I have chosen to place a haitus on all of my fics here on FFN. This is not a permanent break, nor is it a very short one. I chose to do this because I am extremely busy lately, and will be for a couple months. I also have become an active beta-reader, both on FFN and for a webcomic, and I want to make sure I have enough time to help the authors and artists that I am beta-reading for. I also went through my fics here, and have made the decision to ask you all wether or not I should continue my fics, rewrite them, or do a mix.

You can each vote on what I should do on the poll that is currently up on my profile. The poll will be open for the next couple weeks, so make sure you vote so I know what you would prefer.

The amount of readers or viewers on all of my fanfictions are awesome, and thank you all for choosing to read my writing. Like I said, my fics aren't dead, but will be updated eventually.

Thank you all, and have a lovely day.

~thederpypikachu98

TL;DR: There's a poll on my profile to see if I should rewrite stuff, I'm busy, stuff won't update for a while.


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